Image sensors, which are semiconductor devices for converting an optical image into an electrical signal, may be categorized as charge coupled device (CCD) image sensors and complementary metal oxide silicon (CMOS) image sensors.
CCDs have disadvantages. These include complicated driving requirements, relatively high power consumption, and a complicated manufacturing process which requires a multi-stage photolithography process. As a next generation image sensor for overcoming the disadvantages of CCDs, CMOS image sensors have attracted interest.
CMOS image sensors use a photo diode and a MOS transistor in each unit pixel. Images are detected by sequentially detecting the electrical signals from each unit pixel. CMOS image sensors according to the related art can be divided into a photo diode region for receiving light signals and changing them into electrical signals, and a transistor region for processing the electrical signals. However, the CMOS image sensor according to the related art uses a structure having the photo diode horizontally arranged with the transistor.
So while the disadvantages of the CCD image sensor have been reduced by the horizontal type CMOS image sensor, the horizontal type CMOS image sensor according to the related art still has problems. In other words, in the planar CMOS image sensor according to the related art, the photo diode and the transistor are manufactured to be horizontally adjacent to each other on the substrate. Accordingly, additional area is required to accommodate both the photo diode and the transistor. As a result, fill factor (the percentage of the area filled by photosensitive regions) may be reduced, and higher resolutions may be limited.
In the planar CMOS image sensor according to the related art, it is very difficult to simultaneously optimize the manufacturing processes of the photo diode and the transistor. For example, in the transistor process, a shallow junction for low sheet resistance is required; however, in the photo diode process, such a shallow junction may not be proper.
In a planar CMOS image sensor according to the related art, as additional on-chip functions are integrated onto the image sensor, the size of the unit pixel may be increased for maintaining the photosensitivity of the image sensor, or the area for the photo diode may be reduced for maintaining a pixel size. If the unit pixel size is increased the resolution of the image sensor is reduced. If the area of the photo diode is reduced to maintain unit pixel size, the photosensitivity of the image sensor is reduced.